L6599ATN datasheet(18/31 Pages) STMICROELECTRONICS

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Application information

L6599AT

18/31

Doc ID 15534 Rev 3

6.3

Soft-start

Generally speaking, the purpose of soft-start is to progressively increase the converter's

power capability when it is started up, so as to avoid excessive inrush current. In resonant

converters the deliverable power depends inversely on frequency, so soft-start is attained by

sweeping the operating frequency from an initial high value until the control loop takes over.

With the L6599AT, the converter's soft startup is easily achieved with the addition of an R-C

series circuit from pin 4 (RFmin) to ground (see Figure 10, left).

Initially, capacitor CSS is totally discharged, so that the series resistor RSS is effectively

parallel to RFmin and the resulting initial frequency is determined by RSS and RFmin only,

since the optocoupler's phototransistor is cut off (as long as the output voltage is not too far

from the regulated value):

Equation 4

The CSS capacitor is progressively charged until its voltage reaches the reference voltage

(2 V) and, consequently, the current through RSS goes to zero. This typically takes 5 times

constants RSS·CSS. However, before this time interval completes, the output voltage comes

close to the regulated value and the feedback loop takes over, so that the optocoupler's

phototransistor determines the operating frequency from that moment onwards.

During this frequency sweep phase the operating frequency decays, following the

exponential charge of CSS. That is, initially it changes relatively quickly but the rate of

change gets slower and slower. This counteracts the non-linear frequency dependence of

the tank circuit that makes the converter's power capability change little as frequency is

away from resonance, and change very quickly as the frequency approaches the resonance

frequency (see Figure 10, right).

Figure 10.

Soft-start circuit (left) and power vs. frequency curve in a resonant

half-bridge (right)

As a result, the average input current increases smoothly, without the peaking that occurs

with linear frequency sweep, and the output voltage reaches the regulated value with almost

no overshoot.

Typically, RSS and CSS are selected based on the following relationships:

()

min

start

⋅

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